Add torchrun launcher plugin

[jbaczek]

Motivation

PyTorch distributed apps is a common use case for hydra (according to many github issues). Currently implementation of torchrun interferes with hydra in a way, that causes hydra to initialize multiple times, leading to many race conditions.
This PR contains a launcher plugin that fixes the issue, forking the process after hydra is initialized.

Have you read the Contributing Guidelines on pull requests?

Yes
This MR is submitted on behalf of NVIDIA.

Test Plan

Use machine with multiple GPUs.
Use pytorch docker conrainer or install PyTorch in any other way.
Install plugin with pip.
How to run torch distributed without plugin:

torchrun --nproc_per_node=8 my_app.py

How to run with plugin:

python my_app.py -m hydra/launcher=torchrun hydra.launcher.nproc_per_node=8

Related Issues and PRs

Resolves #2038

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[Jasha10]

Thanks @jbaczek.
This idea of wrapping the task function is an interesting approach to solving the problem of launching multiple processes for a single hydra run (context: #2038 comment).

[pixelb]

We're considering having a contrib/ dir to serve as a staging area for various new launchers / sweepers.
Re failing tests, it looks like you just need to run black over the new sources.
We'll do the necessary to merge it to contrib/.

Thank you for your contribution!

[jbaczek]

Should I lint it? Or will you do this on your side?

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[jieru-hu]

hi @jbaczek - thanks very much for your contribution!

We've created contrib here for plugin contributions. We've also included a README there.
Could you update this PR and move this plugin to the contrib folder and add integration tests?

pls let us know if you have any question on this. thanks again.

[jbaczek]

I'll update it as soon as I'll have some time for it. Probably mid-may

[jbaczek]

@jieru-hu I've moved the plugin to the contrib dir and added the tests. Do you need me to do something more?

[jieru-hu]

thanks @jbaczek and sorry for the late response. I will take a look at this

[jieru-hu]

looks great, thanks for your contribution @jbaczek !